The use of thermodynamic techniques for converting heat energy into mechanical, electrical, or some other type of energy has a long history. The basic principle by which such techniques function is to provide a large temperature differential (or the high enthalpy difference gradient) across a thermodynamic engine and to convert the heat represented by that temperature differential into a different form of energy.
Aspects of the invention use atmospheric air as resource of renewable energy through the unique evaporative cooling process to create the enthalpy difference gradient for producing power through the regenerative piston engine. The basic operating principles of the embodiments of the invention involve simultaneously utilizing atmospheric air and water and as its phase changing in the subatmospheric thermodynamic cycle using for it of small account of heat.
The known process of evaporation in dry air permits one, in theory, to extract energy by an «atmospheric engine» from atmospheric air (the water-air system) in form of useful work through difference temperatures or enthalpy difference. One such «atmospheric engine» is a simple toy called the “drinking bird” that can be found in almost any novelty shop. This engine is a closed cycle condensing heat engine and uses the ambient environment as its high temperature heat reservoir; it operates by generating an artificial low temperature heat reservoir by evaporating water. The “drinking bird” heat engine operates on the temperature difference between the ambient temperature (dry bulb) and the wet bulb temperature of outside air. When the “bird” is in the vertical position and the head is wet, vapor in the warmer lower bulb forces fluid up to the cool head through a tube. As the head gets full, the bird gets top-heavy and topples into the horizontal position. In this position, the lower end of the aforementioned tube comes out of the liquid, permitting the head to drain and hence the bird to right itself and repeat the cycle.
The basic thermodynamic operating principles of this “drinking bird” heat engine were analyzed by Carl Bachhuber in his paper, “Energy from the Evaporation of Water,” American Journal of Physics, Vol. 51, No. 3, March 1983, pp. 259-264. In particular, Bachhuber has shown that ordinary water can be used to extract an unlimited amount of natural thermal energy from the surrounding environment by creating enthalpy difference gradient and convert it into mechanical work. Moreover, the specific energy of the water evaporative process than can be converted into useful mechanical work by this «atmospheric engine» is approximately twice the specific energy available in automotive storage batteries. In a technical report: «A Simple Heat Engine of Possible Utility in Primitive Environments», Rand Corporation Publication No. P-3367 (August 1966), Richard Murrow proposed constructing larger versions of this «atmospheric engine» for pumping water from the Nile River. A scaled up model of the basic drinking bird engine was constructed to a height of seven feet and found to be able to generate considerable mechanical work. (See, “The Research Frontier—Where is Science Taking Us,” Saturday Review, Vol. 50, Jun. 3, 1967, pp. 51-55, by Richard Murrow.) Obviously, engines such as these are not “perpetual motion machines.” In principle, larger engines of this type could be used to propel ocean going vessels indefinitely using ordinary sea water for generating an unlimited amount of mechanical work. Although this possibility is generally believed to be thermodynamically impossible, it is clearly not impossible. The existence of these «atmospheric engine» proves that it is indeed possible to convert natural heat energy of the environment at ambient temperature into an unlimited amount of mechanical work by creating an artificial low temperature heat reservoir below ambient through the evaporative cooling process.
The “atmospheric engine” disclosed herein is a semi-open cycle, multi-stage, heat engine that also converts natural ambient heat energy of the environment into mechanical work but uses ordinary air instead of water to create an artificial low temperature heat reservoir. Since air is universally available all over the Earth, the atmospheric engine will be much more practical than the drinking bird engine. It will be shown that the specific energy of air that can be converted into mechanical work by the atmospheric engine is much higher than the specific energy of water used in the drinking bird engine. Hence, the disclosed atmospheric engine will be much more powerful than the drinking bird engine. Since the disclosed atmospheric engine is not a closed cycle engine and operates, as in the case of the drinking bird engine, by generating the enthalpy difference gradient through an artificial low temperature heat reservoir below ambient by evaporative cooling process, it does not violate the second law of thermodynamics.
A team of bioengineers led by Ozgur Sahin at Columbia University have just created the world's first evaporation-driven engine. This method and system for providing an engine for producing mechanical energy through the condensation and evaporation of moisture uses a hygroscopic material in one or more configurations to do mechanical work. The hygroscopic material can include microbial spores or other materials that absorb moisture and expand or swell when exposed to high relative humidity environments and shrink or return to nearly their original size or shape when exposed to low relative humidity environments wherein the moisture evaporates and is released. But this engine is as the slight toy (less than four inches) and has small thermal efficiency, because it uses only of the known tiny temperature gradients between dry bulb and wet bulb temperatures.
Although these particular designs for extracting work from the water-air system may be impractical, the theoretical limit for the work that can be extracted from the water-air system is impressive if the air is hot and dry. At 10% relative humidity and body temperature, the available energy per unit mass of water is about twice the available energy per unit mass of an automotive battery.
The known “atmospheric engines” aren't thermal efficient because they operate on the temperature difference between the ambient temperature (dry bulb) and the wet bulb temperature of outside air. Usually theses difference temperatures are small magnitudes which are defined of small degreases of the enthalpy difference gradients for power (work) generation through engines.
The output work of any engine can be significantly increased by using the Maisotsenko Cycle (see «Life below the wet bulb: The Maisotsenko cycle», POWER, November/December 2003, pp. 29-31). The Maisotsenko Cycle operates on the larger temperature difference between the ambient temperature (dry bulb) and the dew point temperature, (not the wet bulb temperature) of outside air. Therefore it is possible to create higher degreases of the enthalpy difference gradients for power (work) generation through the piston engines.
Typically, the heat differential is provided by hydrocarbon combustion, although the use of other techniques is known. Using such systems, power is typically generated with an efficiency only of about 30%, although some internal-combustion engines have efficiencies as high as 50% by running at very high temperatures and pressure. The efficiency of the existing running combustion engines to convert heat from the ambient environment into the mechanical form of energy may sometimes be less than 10%.
It is well known that one of the disadvantages of presently known combustion engines is inefficiency. Another problem is the requirement of materials which will not only withstand high pressures but also high pressures at high temperatures, particularly in gases and combustion products which tend to be corrosive. Efforts to overcome these problems usually result in solutions involving considerable cost penalties, so that efficient combustion engines remain unavailable to the general public.
Conversion of heat into mechanical energy is typically achieved using the piston engines like an Otto, Diesel or Stirling engines, which implement a Carnot cycle to convert the thermal energy. The mechanical energy may subsequently be converted to electrical energy using any of a variety of known electromechanical systems.
Conventional Otto and Diesel cycle piston engines operate by in taking fresh air, compressing the air, burning fuel with the air to produce high temperature, high pressure combustion products, expanding the combustion products to convert part of the heat into work, and then exhausting the spent gases. These engine cycles are relatively inefficient in converting heat into work because of the practical limitations on the extent of the compression and expansion processes. In order to operate at the highest efficiency which is theoretically possible, the hot product gases would need to be expanded until they had cooled to room temperature. However, with the thermal properties of ordinary combustion products, this would require an expansion volume ratio in excess of one thousand. Those skilled in the art will be aware that an engine with a volume expansion ratio of one thousand and a final gas pressure at or near atmospheric would require a peak pressure so high as to threaten destruction of cylinder, piston and other moving parts, and would aggravate losses from friction, heat transfer, and gas leakage. Also using high temperatures and high pressure for conventional power cycles, this leads to the significant irreversible losses which greatly reduce the thermal efficiency of these power systems.
The embodiments of the invention use the renewable energy to create of the high enthalpy difference gradient of the working fluid for producing power through the double-acting piston engine. Energy conversion system for deriving of useful power by the double-acting piston engine from sources of renewable energy was proposed in 1979 by Charles Jahnig through the U.S. Pat. No. 4,170,878. However, this engine requires enormous surface areas because the operation inherently has very low conversion efficiency, typically 3 to 5%, and so enormous amounts of heat must be transferred. Moreover, this heat must be transferred at very small temperature differences, such as 2° F. to 5° F. Therefore this double-acting piston engine has small thermal efficiency because it cannot create of the high enthalpy difference gradient of the working fluid.
The embodiments of the invention use atmospheric air as resource of renewable energy through the unique evaporative cooling process to create enthalpy difference gradient for producing power. The first time the direct evaporative cooling process was proposed for the piston Stirling engine by Cool Energy Inc in 2007 through the U.S. Pat. No. 7,694,514. Here a hot water is injected into the working fluid in a working space of a hot region and cold water is injected in a working space of a cold region of the piston Stirling engine. Therefore realize better isothermal heat addition and heat rejection processes within the piston Stirling engine. The most interesting conception through the evaporative cooling process for the piston Stirling engine was proposed by Weavers through the U.S. Pat. No. 7,810,330: “Power generation using thermal gradients maintained by phase transitions”. It makes use of water-air phase transitions through the direct evaporative cooling process to maintain a thermal gradient in driving an engine to convert the heat into mechanical energy in power-generation applications. These known evaporative cooling technologies have increased thermal efficiency of the piston engine only about 1.5-2%.
A very promising technique for substantially increasing the thermal efficiency of a combustion piston engines is through thermal regeneration. Thermal regeneration, as used herein, implies the capture of exhaust gas heat from one engine cycle and the transfer of this heat to the working fluid of the subsequent cycle following its compression, but prior to the combustion of the fuel, so as to reduce the required quantity of fuel to be burned. A number of attempts have been made to devise means by which regenerative features similar to those employed in the piston Stirling or Ericsson type engine could be used to accomplish this. Most notable of these piston techniques are those of Hirsch (1874, U.S. Pat. No. 155,087), Martinka (1937, U.S. Pat. No. 2,239,922), Pattas (1973, U.S. Pat. No. 3,777,718), Bland (1975, U.S. Pat. No. 3,871,179), Pfefferle (1975, U.S. Pat. No. 3,923,011), Cowans (1977, U.S. Pat. No. 4,004,421), and Stockton (1978, U.S. Pat. No. 4,074,533).
Analyses of regenerative engine cycles show that the idealized thermodynamic thermal efficiency increases when the compression ratio is decreased, however the cycle mean effective pressure and specific power output decreases as the compression ratio is decreased. Since an engine with low cycle effective pressure must be larger for a given power output, and since heat conduction losses and friction losses increase with engine size, an optimum design must have an intermediate value for compression ratio, i.e. it must be low enough to give acceptable thermodynamic efficiency, but high enough to give low heat conduction and friction losses.
All conventional regenerator designs are the same, or very similar Usually they are the plate or shall and tube heat exchangers, where there is the heat exchange mechanism through surface between the hot and cold working fluids. Heat exchange process between these fluids (usually it is air or gas or their mix) is so small but pressure drop is pretty big. This is explained that those working fluids for any existing engines are one phase flows, which don't change their state of aggregation during of implementation of the power cycle, always remaining in the gas phase.
A prior art, an example of the atmospheric combustion engine is disclosed in the Japanese patent, JP 2002-242700 A. Here expands a high-temperature gas of the atmospheric pressure produced by atmospheric combustion, recovers heat from the gas by a regenerative heat exchanger and a cooler, and sucks, pressurizes and discharges the gas by a compressor. Latter this atmospheric combustion engine was improved by Tanaka (see U.S. Pat. No. 7,204,077) increasing of the power generating efficiency from 28, 1% to 33, 5%. But anyway this system is so complicated and not enough efficient.
Some atmospheric combustion engines have been proposed for increasing of the power generating efficiency. For example, such solar thermally driven power system comprises a solar air heater for focusing solar radiation. Air within the solar heater is heated generally at atmospheric pressure by heat absorption and the heated air is supplied through a humidifying air recuperator to a rotatable turbine of an atmospheric pressure turbine system as a power generating device. This power system, using solar energy, can be realized only together with a rotatable turbine.
Therefore, the existing piston power generation methods and engines are not thermal efficient and cannot be realized for the regenerative piston engine. The known piston power systems and engines don't provide a means through the Maisotsenko Cycle for humidifying and heating of the airflow for the expansion process of the regenerative piston engine in a thermodynamically efficient manner and consequently cannot guarantee small level of density through high level of moisture and temperature for this air. It is known that airflow with higher absolute humidity and temperature (high enthalpy) for the expansion process increases the thermal efficiency of the piston engine. Besides, the known piston power systems cannot guarantee a high level of density through small absolute humidity and temperature for this airflow, using efficient cooling process. It is known that airflow with a small absolute humidity and temperature (small enthalpy) for the compression process increases the thermal efficiency of the piston engine.
Difference of enthalpies (or difference of densities) of the working fluid are a driving force for the proposed subatmospheric regenerative piston engine. Enthalpy change or enthalpy difference gradient is the name given to the amount of heat evolved or absorbed in a reaction carried out at constant pressure. It is given the symbol ΔH, read as “delta H”. It is known from thermodynamics for any adiabatic process the technical or mechanical work (W) done by engine is enthalpy difference (ΔH) of the working fluid at the beginning (H1) and end (H2) of the process: W=H1−H2=ΔH. Thereby growing of a value of enthalpy difference (ΔH) this leads increasingly to boost of the mechanical work (W) done by engine and hence to increasing of the thermal efficiency of a power engine.